1. Field of the Invention
This invention relates to a power supply circuit, and in particular to a switching regulator for converting an alternating current voltage into a regulated direct current voltage to be used as a source of electrical energy for driving a desired electrical circuit.
2. Description of the Prior Art
In general, in a power supply circuit for converting a commercial line alternating current power source into a d.c. voltage, use is usually made of a transformer to either step up or step down the voltage level of the commercial line power source and/or to electrically isolate the output terminal of the power supply circuit from its input terminal to which the commercial line power is applied. In order to make such a transformer to be used in a power supply circuit smaller in size and lighter in weight as well as to enhance the power efficiency of power supply circuit, there has recently been proposed a switching regulator according to which a commercial line voltage is applied to the primary winding of a pulse transformer through a switching circuit and the voltage induced at the secondary winding is applied to the output terminal after having been suitably rectified and filtered whereby the switching circuit is turned on or off with the timing determined by the output voltage at the output terminal so that a constant d.c. output voltage may be obtained at the output terminal.
However, in prior art switching regulators, the switching operation takes place once in each positive or negative half cycle of the alternating commercial line voltage, so that the period of switching is relatively long. For this reason, the filter circuit connected to the secondary winding of the pulse transformer cannot be made compact in size. Moreover, since the transmission of power from the primary side to the secondary side of the pulse transformer is limited by the once-for-each-half cycle switching time period, i.e., the time period while the primary side is on, an attempt to obtain a relatively large output power necessarily makes the switching time period longer, which, in turn, makes an exciting current at the primary side larger thereby precluding the possibility of using an even smaller transformer to make the whole device appropriately compact in size.